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Empirical Constraints on Trojan Companions and Orbital Eccentricities in 25 Transiting Exoplanetary Systems
We present a search for Trojan companions to 25 transiting exoplanets.We use the technique of Ford & Gaudi, in which a difference issought between the observed transit time and the transit time that iscalculated by fitting a two-body Keplerian orbit to the radial-velocitydata. This technique is sensitive to the imbalance of mass at the L4/L5points of the planet-star orbit. No companions were detected above2σ confidence. The median 2σ upper limit is 56 M⊕, and the most constraining limit is 2.8 M⊕ for the case of GJ 436. A similar survey usingforthcoming data from the Kepler satellite mission, along with theradial-velocity data that will be needed to confirm transit candidates,will be sensitive to 10-50 M ⊕ Trojan companions in thehabitable zones of their parent stars. As a by-product of this study, wepresent empirical constraints on the eccentricities of the planetaryorbits, including those which have previously been assumed to becircular. The limits on eccentricity are of interest for investigationsof tidal circularization and for bounding possible systematic errors inthe measured planetary radii and the predicted times of secondaryeclipses.
| Falling Transiting Extrasolar Giant Planets
We revisit the tidal stability of extrasolar systems harboring atransiting planet and demonstrate that, independently of any tidalmodel, none, but one (HAT-P-2b) of these planets has a tidal equilibriumstate, which implies ultimately a collision of these objects with theirhost star. Consequently, conventional circularization andsynchronization timescales cannot be defined because the correspondingstates do not represent the endpoint of the tidal evolution. Usingnumerical simulations of the coupled tidal equations for the spin andorbital parameters of each transiting planetary system, we confirm thesepredictions and show that the orbital eccentricity and the stellarobliquity do not follow the usually assumed exponential relaxation butinstead decrease significantly, eventually reaching a zero value onlyduring the final runaway merging of the planet with the star. The onlycharacteristic evolution timescale of all rotational and orbitalparameters is the lifetime of the system, which crucially depends on themagnitude of tidal dissipation within the star. These results imply thatthe nearly circular orbits of transiting planets and the alignmentbetween the stellar spin axis and the planetary orbit are unlikely to bedue to tidal dissipation. Other dissipative mechanisms, for instanceinteractions with the protoplanetary disk, must be invoked to explainthese properties.
| Extrasolar Giant Planets and X-Ray Activity
We have carried out a survey of X-ray emission from stars with giantplanets, combining both archival and targeted surveys. Over 230 starshave been currently identified as possessing planets, and roughlyone-third of these have been detected in X-rays. We carry out detailedstatistical analysis on a volume-limited sample of main-sequence starsystems with detected planets, comparing subsamples of stars that haveclose-in planets with stars that have more distant planets. Thisanalysis reveals strong evidence that stars with close-in giant planetsare on average more X-ray active by a factor of ~4 than those withplanets that are more distant. This result persists for various sampleselections. We find that even after accounting for observational samplebias, a significant residual difference still remains. Thisobservational result is consistent with the hypothesis that giantplanets in close proximity to the primary stars influence the stellarmagnetic activity.
| Homogeneous studies of transiting extrasolar planets - I. Light-curve analyses
I present a homogeneous analysis of the transit light curves of 14well-observed transiting extrasolar planets. The light curves aremodelled using JKTEBOP, random errors are measured using Monte Carlosimulations and the effects of correlated noise are included using aresidual-permutation algorithm. The importance of stellar limb darkeningon the light-curve solutions and parameter uncertainties is investigatedusing five different limb darkening laws and including different numbersof coefficients as fitted parameters. The linear limb darkening lawcannot adequately fit the Hubble Space Telescope (HST) photometry ofHD209458, but the other four laws give very similar results to eachother for all transit light curves. In most cases fixing the limbdarkening coefficients at theoretically predicted values does not biasthe results, but does cause the error estimates to be too small. Theavailable theoretical limb darkening coefficients clearly disagree withempirical values measured from the HST light curves of HD209458 limbdarkening must be included as fitted parameters when analysinghigh-quality light curves.In most cases the results of my analysis agree with the values found byother authors, but the uncertainties I find can be significantly larger(by factors of up to 3). Despite these greater uncertainty estimates,the analyses of sets of independent light curves for both HD189733 andHD209458 lead to results which do not agree with each other. Thisdiscrepancy is worst for the ratio of the radii (6.7σ for HD189733and 3.7σ for HD209458), which depends primarily on the depth ofthe transit. It is therefore not due to the analysis method but ispresent in the light curves. These underlying systematic errors cannotbe detected from the reduced data alone unless at least threeindependent light curves are available for an individual planetarysystem.The surface gravities of transiting extrasolar planets are known to becorrelated with their orbital periods. New surface gravity values,calculated from the light-curve results and the stellar spectroscopicorbits, show that this correlation is still present. New high-precisionlight curves are needed for HD149026, OGLE-TR-10, OGLE-TR-56,OGLE-TR-132 and GJ436, and new radial velocity curves for the XO-1,WASP-1, WASP-2 and the OGLE (Optical Gravitational Lensing Experiment)planetary systems.
| Theoretical Spectra and Light Curves of Close-in Extrasolar Giant Planets and Comparison with Data
We present theoretical atmosphere, spectral, and light-curve models forextrasolar giant planets (EGPs) undergoing strong irradiation for whichSpitzer planet/star contrast ratios or light curves have been published(circa 2007 June). These include HD 209458b, HD 189733b, TrES-1, HD149026b, HD 179949b, and υ And b. By comparing models with data, wefind that a number of EGP atmospheres experience thermal inversions andhave stratospheres. This is particularly true for HD 209458b, HD149026b, and υ And b. This finding translates into qualitativechanges in the planet/star contrast ratios at secondary eclipse and inclose-in EGP orbital light curves. Moreover, the presence of atmosphericwater in abundance is fully consistent with all the Spitzer data for themeasured planets. For planets with stratospheres, water absorptionfeatures invert into emission features and mid-infrared fluxes can beenhanced by a factor of 2. In addition, the character of near-infraredplanetary spectra can be radically altered. We derive a correlationbetween the importance of such stratospheres and the stellar flux on theplanet, suggesting that close-in EGPs bifurcate into two groups: thosewith and without stratospheres. From the finding that TrES-1 shows nosigns of a stratosphere, while HD 209458b does, we estimate themagnitude of this stellar flux breakpoint. We find that the heatredistribution parameter, Pn, for the family of close-in EGPsassumes values from ~0.1 to ~0.4. This paper provides a broadtheoretical context for the future direct characterization of EGPs intight orbits around their illuminating stars.
| Improved Parameters for Extrasolar Transiting Planets
We present refined values for the physical parameters of transitingexoplanets, based on a self-consistent and uniform analysis of transitlight curves and the observable properties of the host stars. Previouslyit has been difficult to interpret the ensemble properties of transitingexoplanets because of the widely different methodologies that have beenapplied in individual cases. Furthermore, previous studies often ignoredan important constraint on the mean stellar density that can be deriveddirectly from the light curve. The main contributions of this work are(1) a critical compilation and error assessment of all reported valuesfor the effective temperature and metallicity of the host stars, (2) theapplication of a consistent methodology and treatment of errors inmodeling the transit light curves, and (3) more accurate estimates ofthe stellar mass and radius based on stellar evolution models,incorporating the photometric constraint on the stellar density. We useour results to revisit some previously proposed patterns andcorrelations within the ensemble. We confirm the mass-period correlationand find evidence for a new pattern within the scatter about thiscorrelation: planets around metal-poor stars are more massive than thosearound metal-rich stars at a given orbital period. Likewise, we confirmthe proposed dichotomy of planets according to their Safronov number,and we find evidence that the systems with small Safronov numbers aremore metal-rich on average. Finally, we confirm the trend that led tothe suggestion that higher metallicity stars harbor planets with agreater heavy-element content.
| SuperWASP-N extrasolar planet candidates from fields 06h < RA < 16h
The Wide Angle Search for Planets (WASP) survey currently operates twoinstallations, designated SuperWASP-N and SuperWASP-S, located in theNorthern and Southern hemispheres, respectively. These installations aredesigned to provide high time-resolution photometry for the purpose ofdetecting transiting extrasolar planets, asteroids, and transientevents. Here, we present results from a transit-hunting observingcampaign using SuperWASP-N covering a right ascension (RA) range of06h < RA < 16h. This paper represents thefifth and final in the series of transit candidates released from the2004 observing season. In total, 729335 stars from 33 fields weremonitored with 130566 having sufficient precision to be scanned fortransit signatures. Using a robust transit detection algorithm andselection criteria, six stars were found to have events consistent withthe signature of a transiting extrasolar planet based on the photometry,including the known transiting planet XO-1b. These transit candidatesare presented here along with discussion of follow-up observations andthe expected number of candidates in relation to the overall observingstrategy.
| The 3.6-8.0 μm Broadband Emission Spectrum of HD 209458b: Evidence for an Atmospheric Temperature Inversion
We estimate the strength of the bandpass-integrated thermal emissionfrom the extrasolar planet HD 209458b at 3.6, 4.5, 5.8, and 8.0 μmusing the Infrared Array Camera (IRAC) on the Spitzer Space Telescope.We observe a single secondary eclipse simultaneously in all fourbandpasses and find relative eclipse depths of 0.00094+/-0.00009,0.00213+/-0.00015, 0.00301+/-0.00043, and 0.00240+/-0.00026,respectively. These eclipse depths reveal that the shape of the inferredemission spectrum for the planet differs significantly from thepredictions of standard atmosphere models; instead, the most plausibleexplanation would require the presence of an inversion layer high in theatmosphere leading to significant water emission in the 4.5 and 5.8μm bandpasses. This is the first clear indication of such atemperature inversion in the atmosphere of a hot Jupiter, as previousobservations of other planets appeared to be in reasonably goodagreement with the predictions of models without such an inversionlayer.
| HAT-P-3b: A Heavy-Element-rich Planet Transiting a K Dwarf Star
We report the discovery of a Jupiter-size planet transiting a relativelybright (V=11.56) and metal-rich early K dwarf star with a period of ~2.9days. On the basis of follow-up photometry and spectroscopy we determinethe mass and radius of the planet, HAT-P-3b, to beMp=0.599+/-0.026 MJup andRp=0.890+/-0.046 RJup. The relatively small sizeof the object for its mass implies the presence of about 75M⊕ worth of heavy elements (~1/3 of the total mass)based on current theories of irradiated extrasolar giant planets,similar to the mass of the core inferred for the transiting planet HD149026b. The bulk density of HAT-P-3b is found to beρp=1.06+/-0.17 g cm-3, and the planet orbitsthe star at a distance of 0.03894 AU. Ephemerides for the transitcenters areTc=2,454,218.7594+/-0.0029+N×(2.899703+/-0.000054)(HJD).
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